The use of certain polymers as resists for lithographic processing of semiconductor substrates is well known. Frequently, the substrate has topographical features which cause a resist coating to be uneven. In order to produce lithographic patterns finer than one micrometer, it is required that a resist pattern layer be smooth and have minimal thickness variations. To achieve this, it is conventional to initially coat the substrate with a spun-on coating of a polymer which planarizes topographical features and apply the resist layer thereover.
The use of multilayer resist systems comprising at least a planarizing layer and a resist layer has not totally solved the problem of how to effectively planarize substrate topographical features, particularly features which are nonuniformly oriented with respect to the centrifugal center of the substrate. It has been demonstrated, for example, that spin-coating, which is the conventionally used technique of coating a substrate with a resist preparation, will produce radial flow anomalies when the resist is applied to a substrate having nonuniform topographical features.
Radial flow anomalies on a substrate having nonuniform topographical features will change merely by varying the orientation of the features with respect to the centrifugal center of the wafer being spin-coated without varying the size or the configuration of the features themselves. These anomalies cause a bulid-up of coating material on the side of the topographical features adjacent to the centrifugal center, increased step heights in the coating and asymmetric coating contours. These effects become more pronounced as the distance from the centrifugal center of the wafer increases. It has further been demonstrated that changing the position or the size of a single topographical feature can affect the thickness of the coating on adjacent features, particularly those situated outwardly from the altered feature on a line extending from the centrifugal center. It will be appreciated that the problems associated with planarizing the surface of a semiconductor substrate having nonuniform topographical features become more complex as design dimensions shrink and circuitry is built vertically from the wafer surface. A means of improving the spin-coating of a solution of an organic material onto a substrate having topographical features is provided in accordance with this invention.